1. Field of the Invention
The present invention relates to the art of papermaking.
More specifically, the present invention relates to the maintenance of paper web carrying screens and felts in a clean and operatively efficient condition.
2. Description of the Prior Art
In the papermaking process, an aqueous slurry of cellulose is laid upon the traveling, table surface of an endless belt of screen material. In transit, from one end of the table surface to the other, a major portion of the water vehicle is drawn through the screen cell spaces to leave a loosely matted web of wet cellulose.
At the dry end of the traveling table, the web mat is transferred to a tightly woven fabric belt characterized as a press felt. One function of the press felt is to provide substrate support for the web as it is threaded through a plurality of roll press nips. Another function of the press felt is as an intimate contact moisture wick to draw additional water from the web.
In the course of continuous operation, the press felt will accumulate minute particles of cellulose fiber to fill, coat or otherwise obstruct the porous quality of the felt. Such coatings and fillings greatly inhibit the wicking function of the felt and streak the web carried thereon.
The conventional technique for cleaning a press felt without removal from service is to intermittently or continuously direct a multiplicity of high pressure jet streams of water against the felt surface. Typically, such jet streams are spaced across the felt width on six or twelve inch centers and reciprocated in the cross direction with an approximately twelve inch stroke. U.S. Pat. Nos. 1,381,272 and 3,135,653 provide representative descriptions of suitable equipment.
Theoretically, each jet stream will clean a narrow band area about the felt circumference. If the traverse rate is coordinated with the felt speed, the entire felt is cleaned over a period of time by a continuous, closely wound helical trace of each jet stream around the felt perimeter.
Multiple jet streams issuing from respective nozzles in a pipe spanning the felt width reduce the oscillation period of the pipe and hence, the cleaning cycle. The helical trace of each jet continues from the stroke start to termination. The terminating point of one jet is the starting point for the next adjacent jet.
There are numerous permutations of this general cleaning scheme but the usual practice is to space the jet nozzles along the conduit length by a distance equal to the oscillation stroke length. An adverse consequence of this scheme is that the bands of shower impact against the felt corresponding to the stroke terminus are flushed twice in a cleaning cycle. Since the felt is minutely damaged by each cycle, these bands of impact coincidence result in areas that are damaged at twice the rate of the remaining felt surface. In time, the damage difference increases sufficiently to mark the paper product with corresponding bands.
Another difficulty with short stroke oscillating showers is that of protecting the oscillation mechanism from a humid, papermachine atmosphere. Although the mechanism may be enclosed by a watertight housing, water eventually invades the interior to foul the machinery. Water invasion is often a result of pressure differentials occurring as a consequence of the stroking mechanism volume which oscillates in and out of the sealed housing chamber. This heretofore unavoidable result has occasioned restricted and expensive mechanical designs using direct current drive motors.
It is therefore, an object of the present invention to provide a felt shower oscillating mechanism which superimposes a movement in the oscillation stroke reference point upon the fixed stroke distance to spread the stroke terminus band over a greater area.
Another object of the invention is to provide a housing for the oscillation mechanism that is served by a regulated low pressure gas supply to exclude environmental air.
Another object of the present invention is to teach the design parameters of a papermachine felt shower oscillating mechanism that may be driven by a low rotor mass, reversible AC motor.